1. Field of Invention
The invention relates to a method for detecting a “misfire” condition in an internal combustion engine.
2. Description of Related Art
In a spark-controlled internal-combustion engine, misfire indicates a condition of poor combustion occurring in one or more cylinders. In particular, the “misfire” condition is referred to as “spark misfire” when it is caused by insufficient or no spark energy generated by the spark plug or “injection misfire” when the condition is caused by insufficient or no fuel feed.
The presence of “misfire” conditions is particularly dangerous because poor combustion causes engine-performance degradation, increase of the level of polluting engine emissions, and possible permanent damage to the catalyst. For this reason, European “emission limit” standards for motor vehicles require to implement effective misfire detection and indicate the presence of “misfire” conditions to the driver by a blinking warning light on the instrument panel. More specifically, European “emission limit” standards require to indicate an increase of the level of polluting emissions when the number of misfires in a first interval [e.g., 1000 “TDC” (top dead centers)] exceeds a first threshold and a permanent degradation of the catalyst when the number of misfires in a second interval (e.g., 1000 “TDC”) exceeds a second threshold.
“Misfire” conditions are currently detected in an indirect manner (i.e., by analyzing either the instantaneous “angular acceleration” value of the driving shaft or instantaneous “driving torque” value because a direct analysis of the combustion by sensors arranged within each cylinder is not feasible due to cost reasons.
The most common method for detecting “misfire” conditions consists in analyzing the angular acceleration of the driving shaft. In particular, by using the signal supplied by the phonic wheel, the value of the angular acceleration of the driving shaft is calculated at given angular positions of the driving shaft itself and “misfire” conditions are detected if the absolute value of the angular acceleration of the driving shaft is higher than a given threshold value.
However, it has been noted that such a method is not very reliable because the absolute value of the angular acceleration of the driving shaft may exceed the threshold value even for causes not connected to “misfire” conditions—such as switching “on” and “off” the air-conditioning-system compressor, road roughness, or sudden decelerations. Furthermore, in case of a single misfire, torsion oscillations may be trigged on the “angular acceleration” value of the driving shaft, which cause the absolute value of the angular acceleration of the driving shaft to exceed the threshold value in several later instants. In this case, a single “misfire” condition is incorrectly detected as a plurality of several successive misfires.
A first solution to the above-described problem is provided by European Patent Application EP-0637738-A1, which describes a method for detecting “misfire” conditions in an internal-combustion engine by analyzing the angular acceleration of the driving shaft. To attempt to eliminate the influence of interferences, the “angular acceleration” value of the driving shaft is processed by creating an index referred to as “cyclicity,” which, compared with appropriate thresholds, indicates the presence of “misfire” conditions. However, the method tends to incorrectly detect a plurality of successive misfires instead of a single misfire due to the oscillations of the angular acceleration of the driving shafts trigged by a single misfire.
European Patent Application EP-A1-1447655 describes a method for detecting “misfire” conditions in an internal-combustion engine by analyzing the angular acceleration of the driving shaft, which includes estimating the “angular acceleration” value of the driving shaft at given angular positions of the driving shaft itself, comparing the absolute value of the angular acceleration of the driving shaft, and detecting the presence of “misfire” conditions if the absolute value of the angular acceleration of the driving shaft is higher than the given threshold value. A series of “angular acceleration” values of the driving shaft, which follow the “angular acceleration” value of the driving shaft at which the “misfire” condition was detected, are filtered to eliminate the oscillation component induced by the “misfire” condition on the “angular acceleration” value of the driving shaft whenever a “misfire” condition is detected (i.e., the absolute value of the angular acceleration of the driving shaft exceeds the threshold value). At this point, only the filtered values of the angular acceleration of the driving shaft are compared with the threshold value to detect the possible presence of further “misfire” conditions that follow the detected one. However, the method proved not to ensure good performance at high engine speeds (in particular, when rpm is higher than 4500).
Thus, it is the object of the invention to provide a method for detecting a “misfire” condition in an internal-combustion engine. More specifically, it is the object of the invention to provide such a method that is free from the above-described drawbacks and, in particular, easy and cost-effective to be implemented and capable of minimizing the percentage of false detections and missed detections of “misfire” conditions.